A device of this type for ion mobility spectroscopy is known from DE 196 27 621 A1. The gas chamber is composed of a drift chamber and a partial chamber of a reaction chamber therein. A thermal electron emitter for generating free electrons is located in a further evacuated partial chamber of the reaction chamber. The free electrons are accelerated with the aid of an electron flux modulator and an acceleration voltage applied between the electron emitter and the electron flux modulator in the direction of an electron-permeable gas-tight membrane between the two partial chambers of the reaction chamber. The membrane is manufactured from mica and is coated on the side facing toward the gas-filled partial chamber of the reaction chamber with a metal layer.
The accelerated electrons enter the partial chamber of the reaction chamber, which is filled with drift gas and analyte gas, through the membrane. The charged gas particles arise in this partial chamber due to the incident electrons. The area of the gas chamber directly neighboring the membrane, in which the incident electrons are absorbed, is therefore to be identified in the following as the charging chamber.
The charging of the gas particles also results in formation of charged analytes. Under the effect of a drift voltage applied between the metallic coating of the membrane and the drift chamber, the charged analytes move from the gas-filled partial chamber of the reaction chamber to a capture electrode situated in the drift chamber and are detected there. Since the different charged analytes have different mobilities and thus different drift velocities, the different analytes require different amounts of time in order to reach the capture electrodes from the charging region placed in front of the membrane.
Ion mobility spectrometers are suitable in particular for detecting organic materials. For example, it is possible to detect organic compounds, such as chemical warfare agents, illegal drugs, or explosive materials in extremely small quantities with the aid of an ion mobility spectrometer.
An advantage of the known ion mobility spectrometer is that it manages without a radioactive source for charging the analytes. Therefore, no special protective measures have to be taken.
However, a disadvantage of the known ion mobility spectrometer is the high thermal strain of the membrane by the electron beam, because of which a support grating is situated on the entry side of the membrane, which mechanically stabilizes the membrane.
The known ion mobility spectrometer is thus not suitable for maintenance-free long-term use. Vice versa, however, there is a need for devices requiring as little maintenance as possible, in order to provide them to fire departments or units of the disaster protection office, for example.
Furthermore, an electron capture detector for gas chromatography, in which electrons generated with the aid of ultraviolet light are emitted through a membrane into a charging chamber, where electron-attracting molecules are charged and detected by collection electrodes, is known from DE 196 27 620 C1.
Moreover, a light source for ultraviolet light is known from U.S. Pat. No. 6,282,222 B1. The known light source uses excimers to generate the ultraviolet light, which are formed in a gas chamber through the effect of an electron beam on noble gas. The electron beam is generated by an electron generator which has an evacuated chamber having a thermal electron emitter. The electrons released by the electron emitter are accelerated in the direction of a metallic anode, which has a central hole which is covered by a carrier. The carrier holds a thin film. The film is preferably produced on the basis of silicon nitride, but may also be produced from a material of the group of carbides, nitrides, hydride, and oxides of metals, which are selected from the group of the elements B, Al, Si. Furthermore, polysilicon is suggested as a material for the membrane. The electron generator is separated from the gas chamber by the membrane.